1. Field of the Invention
The present invention relates to a transmitting device and a receiving device of a wireless orthogonal frequency division multiplex (OFDM) communication system with space time transmit diversity (STTD). Further, the present invention relates to a channel estimation method for performing a channel estimation in a wireless OFDM communication system in which a transmitting device comprising a first and a second antenna transmits signals with space time transmit diversity.
2. Description of the Related Art
In wireless telecommunication, the transmission quality between a transmitting device, such as a base station, and a receiving device, such as a mobile terminal, depends strongly on the respective transmission environment and is often deteriorated by fading effects and the like. This often leads to poor speech and data transmission quality, particularly if only one single antenna is used on the transmission side and one single antenna is used on the receiving side. Therefore, some mobile terminals for wireless telecommunication systems, such as the GSM system, comprise two or more and different kinds of antennas built as internal or external antennas in the mobile terminal. However, it is desirable that modern mobile terminals are as small and light as possible and therefore it is an increasing interest to use only a single antenna in these mobile terminals. In order to allow the use of only a single antenna on the receiving side, particularly the mobile terminal side, it has been proposed to use more than one antenna on the transmitting side, particularly the base station side, so that the diversity gain can be used for a better transmission quality. This scheme is called transmit diversity. Transmit diversity generally means that more than one antenna, e.g. two antennas, transmit data simultaneously to a receiving device. If the same data are transmitted in parallel by two antennas, the receiving side has a chance to receive signals at least from one of the antennas with an acceptable transmission quality so that a good connection can be ensured. One specific approach in the transmit diversity scheme is the use of a so-called space time coding. The resulting space time transmit diversity (STTD) has been adapted and is part of the UMTS standard for the next generation of mobile telecommunication.
In a space time transmit diversity system, a transmitting device, such as a base station, comprises e.g. two antennas arranged spaced apart from each other in a space diversity arrangement. A stream of data to be transmitted to a receiving device, such as a mobile terminal, is encoded and processed so that two parallel data streams are generated.
The proposed scheme of transmitting, receiving and processing first and second pilot symbols allows a simple and effective channel estimation processing to be performed on the receiving side so that a better coherent demodulation of the transmission channel can be performed to ensure the best transmission quality. Particularly, the present invention ensures full space and time diversity. Further, no feedback information from the receiving side to the transmitting side is required and an improved data transmission capacity can be realised. Further, the proposed system is robust to transmission antenna failures and guarantees power amplifier balance on the transmitting side.
Further advantageous features are claimed in the respective subclaims.
Advantageously, the corresponding first and second pilot symbols transmitted from the first and the second antenna means of the transmitting device, respectively, have the same frequency and time allocation in the OFDM system. In other words, corresponding first and second pilot symbols are transmitted in the same subcarrier and in the same time slot of the OFDM system. Hereby, the first and the second pilot symbols advantageously are alternatively identical and orthogonal to each other in the frequency as well as in the time dimension. This means that in the frequency and time grid of the OFDM system, identical first and second pilot symbols and orthogonal first and second pilot symbols alternate with each other in the frequency as well as in the time dimension.
It has to be clarified at this point that the single antenna of the receiving device receives the first pilot symbols transmitted from the first antenna means and the second pilot symbols transmitted from the second antenna means of the transmitting device only as a combined or superimposed pilot symbol. In case that the first pilot symbol and the second pilot symbol transmitted in the same frequency subcarrier and the same timepoint are identical, the receiving device receives a combined pilot symbol comprising the superimposed identical first and second pilot symbol. In case that the first and second pilot symbol are orthogonal to each other, the receiving device receives a combined pilot symbol comprising the superimposed orthogonal first and second pilot symbol. In the receiving device, the transfer function of the first and the second pilot symbol, respectively, can therefore be separated so that the respective channel estimation for each of the two transmission antennas can be performed in a simple way.
Advantageously, the second pilot symbols alternately have the identical and the inverse complex value of the corresponding first pilot symbol in the time as well as in the frequency dimension, so that the processing and the channel estimation on the receiving side can be performed on a basis of a simple addition and subtraction calculation of the received pilot symbols. On the basis of the channel estimation result, both the STTD encoded signals from the first antenna means and from the second antenna means of the transmitting device are further processed and used as the communication data in the receiving device.
The transmitting device according to the present invention can e.g. be implemented in the base station of a OFDM communication system and the receiving device according to the present invention can e.g. be implemented in a mobile terminal of a OFDM communication system.